High q tunable if transformer coil assembly

ABSTRACT

In a conventional tunable transformer coil assembly a coil is wound on a core, and a magnetic tuning cup is movable relative to the core along an axis parallel to the axis of the core to vary the inductance of the transformer. In the present invention, the axis along which the cup moves is substantially at right angles to the axis of the core. This construction provides a coil having a higher value of Q.

United States Patent John Chesney Roselle Park, NJ.

Aug. 13, 1969 Aug. 24, 1971 General Instrument Corporation Newark, NJ.

inventor Appl. No. Filed Patented Assignee HIGH Q TUNABLE IF TRANSFORMER COIL ASSEMBLY 1 Claim, 5 Drawing Figs.

us. Cl 336/83, 336/135, 3315/1216 in. CI non 21/06 Field or Search 336/83,

Primary ExaminerThomas J. Kozma Anorney-James & Franklin ABSTRACT: In a conventional tunable transformer coil assembly a coil is wound on a core, and a magnetic tuning cup is movable relative to the core along an axis parallel to the axis of the core to vary the inductance of the transformer. In the present invention, the axis along which the cup moves is substantially at right angles to the axis of the core. This construction provides a coil having a higher value of Q.

PRIOR ART r II 1 5 5 INVENTOR JOHN CHESNE Y ATTORNEY HIGH Q TUNABLE IF TRANSFORMER COIL ASSEMBLY The present invention relates to an improved, high Q magnetic core transformer coil assembly having particular use in tuning circuits.

Coils having magnetic cores are widely used in high frequency circuits particularly in the IF stages of a communications receiver. It is customary to operatively connect a fixed capacitor either in series or in parallel with the coil to produce a resonant circuit and to vary the coil inductance to tune that circuit to the desired resonant frequency.

In a conventional coil of this type, a winding is wound about a magnetic core or bobbin, and a magnetic tuning member is moved relative to the core to vary the inductance of the coil and thus adjust the resonant frequency.

In resonant tuning circuits of this type it is almost always desirable to obtain the highest possible Q or figure of merit for the coil so as to obtain the maximum selectivity of the tuning circuit. The value of the Q of the coil varies inversely with the energy losses in the coilthe higher the energy losses the lower is the Q of the coil. The coil losses are commonly expressed as an equivalent coil resistance, and the Q is defined as the ratio of the reactance wL of the coil to this equivalent resistance, where M=21rf, and f is the frequency at which Q is being measured.

In the conventional resonant circuit coil, the coil winding is arranged about a core and the tuning member, often in the form of a cup, is movable with respect to the core along an axis which is substantially parallel to the core axis. The magnetic field produced by the current flow in the coil winding extends longitudinally' along the axisof the core and is thus spread out between the ends of the core. This spreading of the magnetic field results in a significantly large decrease in the Q of the coil. This in turn limits the maximum value of Q of the coil and thus limits the selectivity of the resonant circuit of which the coil is a part. I

It is a general object of the present invention to provide a coil having a higher as compared to the prior art coils.

It is another object of the present invention to provide a compact IF transformer coil for use in a resonant circuit, in which the Q and thus the selectivity of that resonant circuit is significantly increased.

It is a further object of the present invention to provide a tunable IF transformer coil having an increased value of Q, which value remains substantially constant over the entire tuning range of the coil.

It is yet another object of the present invention to provide a tunable IF transformer coil having an increased value of Q, in which the inductance of the coil varies substantially linearly over most of its tuning range.

It is still a further object of the present invention to provide a tunable IF transformer coil having a significantly higher value of 0 than the conventional coils of this type and which is substantially the same size as, and is fabricated as readily and as economically as the conventional coils.

As described above, in the conventional coil, the axis of the core or bobbin is substantially parallel to the axis along which the tuning cup moves to vary the coil inductance. In the coil of the present invention, the axis of the core is at substantially a right angle to the axis of the cup. It has been found that by this novel arrangement of the core and cup axes energy losses are reduced, and as a result, the Q of the coil is significantly in creased.

The core is arranged with its major axis disposed transversely in the coil housing and comprises a central portion, upon which the coil is wound, and a pair of radially enlarged end portions. The cup is movable vertically in the housing relative to the core end portions and at its lowermost position encloses most, if not all, of the core. The coil housing comprises an insulating member having a base and spaced parts projecting upwards therefrom in the direction of movement of the cup. These insulating parts are received between the core end portions and at least a portion of the cup for most of the tuning range of the coil assembly, that is, for most positions of the tuning cup along its vertical axis of direction of movement with respect to the core.

To the accomplishment of the above, and to such other ob jects as may hereinafter appear, the present invention relates to a high Q, tunable IF transformer coil assembly, as defined in the appended claims, and as described in the specification, taken together with the accompanying drawings, in which;-

FIG. 1 is a vertical cross-sectional view of a typical prior art coil assembly;

FIG. 2 is a cross-sectional view of a coil assembly illustrating features of the present invention;

FIG. 3 is a cross-sectional view taken along the line 3-3 of FIG. 2;

FIG. 4 is a cross-sectional view taken along the line 4--4 of FIG. 2; and

FIG. 5 is a graphical representation illustrating the relationship between Q and the coil inductance as a function of the,

relative axial position of the movable cup.

FIG. 1 illustrates a conventional tunable IF transformer coil assembly 10 in which a coil 12 is wound about a core or bobbin 14 made of a suitable magnetic material such as powdered iron. The core 14 and the coil 12 are preferably within a shield can or housing 16. A cup 18 made of a suitable magnetic material is also contained within housing 16, is positioned in the magnetic field of coil 12, and is movable vertically with respect to the core 14, thereby to vary the inductance of the coil by varying the effective permeability of the core.

Core 14 rests on an insulating base 20 which includes a pair of spaced inner walls 22 which define a seat for core 14 and coil 12, and a pair of spaced, threaded outer walls 24, which threadably engage the similarly threaded outer periphery of the cup 18. Vertical movement of cup 18 is effected by rotating the cup, as by inserting a screwdriver tip into a slot 26, and rotating the screwdriver to cause cup 18 to rotate and move vertically along with respect to core 14.

Terminal pins 28 project through and extend from base 20 and a suitable electrical connection is made between the two ends of coil 12 and selected ones of the pins 28. If desired, a recess 30 may be formed in the underside of base 20, and a capacitor 32 may be received in the recess. Leads (not shown) extend from capacitor 32 and are suitably connected to the coil 12. Those connections also preferably retain capacitor 32 within recess 30.

It will be noted that in the conventional coil assembly shown in FIG. 1, the vertical relative movement of cup 18 is along an axis which is substantially parallel to the axis of the core, the coil 12 being wound about the latter axis. In this coil assembly the fiux path produced by the flow of current through coil 12 travels between the ends of core 14 and spreads out circumferentially about the axis of core 14. This spreading causes an energy loss in the magnetic circuit of the coil, and for the reasons set forth above, causes a decrease in the net Q of the coil.

The tunable coil assembly of the present invention, as shown in FIGS. 2-4, comprises the same basic elements of the prior art assembly, that is, the core, the coil wound about the core, and a cup located in the magnetic field of the coil and movable with respect to the core. However, in a distinct departure from the prior art construction, the axis of the core of the coil assembly of FIGS. 2-4 is substantially perpendicular to the axis along which the cup moves to tune the coil assembly. It has been found that this construction produces a coil assembly producing a significantly higher value of Q than that of the prior art assembly primarily as a result of the significant reduction of the magnetic energy losses.

In the embodiment of the present invention herein specifically disclosed, the core 34, made of suitable magnetic material such as ferrite or Carbonyl E iron, is supported on an insulating base 36. A coil 37 is wound about the axis of the core 34 and produces a magnetic field when current is passed therethrough. Base 36 comprises a' pair of outer laterally spaced, internally threaded outer walls 38 and 40, and a pair of laterally spaced inner walls 42 and 44. Core 34 is snugly received within the space between inner walls 42 and 44. The tuning slug or cup 46, also made of a magnetic material such as ferrite, has a circular base 48 and a cylindrical externally threaded hollow wall depending therefrom which defines, when viewed in section as in FIG. 2, a ringlike section 50 extending at least in part into the magnetic field produced by coil 37. A screwdriver receiving slot 54 is centrally formed in base48. The externally threaded ringlike section 50 of cup 46 mates with the rotationally threaded outer walls 38 and 40 on insulating base 36. The cup 46 is thus relatively movable along a vertical axis 55 with respect to the walls of insulating base 36 and core 34 by rotating the cup about that vertical axis. The coil assembly is contained within a shield can 16 and, as described with reference to the prior art embodiment of FIG. 1, terminal pins 28 may be passed through base 36 and a capacitor 32 may be placed in a recess 30 formed in the underside of base 36.

The core 34 comprises a central portion 56 and larger radial end portions 58 and 60, an annulus being defined between these end portions. The major axis 61 of core 34 passes centrally through the core end portions 58 and 60 and the central portion 56 and thus, as shown in FIG. 2, is oriented substantially horizontal within the coil assembly and intersects axis 55 at substantially a right angle. Coil 37 is wound about axis 61 and substantially fills the annulus or space defined between the core end portions 58 and 60. The magnetic field produced by the current flowing through the coil creates a flux path which extends between the core end portions and passes through the magnetic material of cup flange section 50 to describe substantially a closed path between the ends of the core.

As the axis of core 34 lies transversely with respect to the cup flange section there is only relatively little spreading of the magnetic flux at right angles to its main direction of travel corresponding to axis 61 as it traverses its path between the core end portions. As a result of this reduced spreading of the flux path, the energy losses in the assembly of FIGS. 2-4 are reduced as compared to the prior art construction shown in FIG. 1.

The base inner walls 42 and 44 are interposed between the core end portions 58 and 60 and the depending cup flange section 50 to provide a relatively small spacing between core 34 and the cup, thereby to provide a nonmagnetic gap between these two magnetic parts. The cup flange section Swhich extends substantially in the direction of the vertical axis 55 along which cup 46 is movable, receives core 34 as well as the upper ends of the base inner walls 42 and 44 therebetween. For most of the available tuning positions of cup 46, the core axis 61, that is, the horizontal axis of the core as viewed in FIG. 3, intersects a portion of the flange section 50.

The improved results obtained by the construction of the coil assembly of the present invention are illustrated in FIG. 5, in which both Q and the ratio of coil inductance to the minimum coil inductance (L/Lmin.) are plotted as a function of the relative axial position of the cup 46. The solid lines of the graph in FIG. represent the values for the coil assembly of the present invention of FIGS. 2-4, and the broken lines illustrate the values obtained for the prior art coil assembly of FIG. 1. As seen in FIG. 5, at a frequency of 10.7 MHz., the

value of Q for the coil assembly of the present invention is substantially constant between 200-220 for substantially all positions of the tuning cup 46. This is to be compared with the lower values of Q for the prior art coil assembly which vary between approximately '160 and for similar axial positions of the tuning cup. Thus anincrease of approximately 15 percent in the value of Q is obtained in the coil assembly of the present invention, as compared to the comparably dimensioned coil assembly of the prior art. Moreover, the values of coil inductance for the coil assembly of the present invention and that of the prior art assembly are substantially the same and vary substantially linearly as a function of cup travel.

The present invention thus provides an improved coil assembly having an increased value of 0 along substantially the entire tuning range of the coil. By the construction of. the present invention coil, assemblies capable of operating f at frequencies of 10.7 MHz. and having Q's of 210-220 are obtained in a IO-mm. square housing configuration with practically a constant Q over the such a higher Q coil assembly in a tuning circuit increases the selectivity of that tuning circuit and thus provides for improved tuning operation of the communications receiver of which that tuning circuit is a part. This is achieved in a very simple manner, as has been explained above, by reorienting the axis of the core and the coil with respect to the axis along which the tuning cup is moved to tune the coil assembly. The resulting assembly may be manufactured at substantially the same cost as the prior art assemblies and with the use of sub stantially the same components. It thus can be readily and economically manufactured in large quantities. Moreover, the size of the coil assembly of the present invention is not greater than that of the prior art coil assemblies so that it can be packaged in as small a space.

While only a single embodiment of the present invention has been herein specifically disclosed, it will'be apparent that many variations can be made thereto without departing from the spirit and scope of the invention.

I claim:

1. A high Q tunable IF transformer coil assembly comprising a base of insulating material, a core of magnetizable but unmagnetized material fixed on said base end arranged along a first axis, a coil fixed on said core and wound about said first axis, said coil being so dimensioned and formed of such wire as to be usable as an inductance of frequencies in the MegaHertz range, and a magnetic member mounted on an adjustably positionable relative to said base, said magnetic member being located in the magnetic field of said coil and movable with respect to said core so as to translate along a second axis, thereby to vary the inductance of said coil, said second axis being substantially perpendicular to said first axis, thereby to increase the effective Q of said coil, said magnetic member comprising a substantially cylindrical structure located radially outside said core and telescopically movable relative thereto translatably along said second axis, the radial space between said cylindrical structure and said core being substantially of uniform width in all operative positions of said cylindrical structure and being free of electrically conductive material, said assembly including structure which, together with said base, substantially completely encloses said coil and core, said coil having leads extending out through said structure to the exterior of said assembly.

entire tuning range. The use of 

1. A high Q tunable IF transformer coil assembly comprising a base of insulating material, a core of magnetizable but unmagnetized material fixed on said base end arranged along a first axis, a coil fixed on said core and wound about said first axis, said coil being so dimensioned and formed of such wire as to be usable as an inductance of frequencies in the MegaHertz range, and a magnetic member mounted on an adjustably positionable relative to said base, said magnetic member being located in the magnetic field of said coil and movable with respect to said core so as to translate along a second axis, thereby to vary the inductance of said coil, said second axis being substantially perpendicular to said first axis, thereby to increase the effective Q of said coil, sAid magnetic member comprising a substantially cylindrical structure located radially outside said core and telescopically movable relative thereto translatably along said second axis, the radial space between said cylindrical structure and said core being substantially of uniform width in all operative positions of said cylindrical structure and being free of electrically conductive material, said assembly including structure which, together with said base, substantially completely encloses said coil and core, said coil having leads extending out through said structure to the exterior of said assembly. 